1. Field of the Invention
The present invention relates to polymeric sulfur compounds, particularly for use in a process of extraction of mercury from aqueous waste streams.
2. Description of Related Art
Due to increased environmental concerns about waste remediation, major efforts are underway to design new materials that can effectively remove and recover toxic metal ions from aqueous solutions. One approach to this problem has been to use organic ligands anchored to solid supports. Such a strategy presents several advantages in the treatment of aqueous waste. First, specific binding properties of the ligands can be exploited to selectively remove a desired metal ion from a complex solution of cations. These materials can also be recycled and reused to make the treatment process cost effective. In addition, this technique can potentially minimize the amount of secondary waste generated in the extraction process, a major challenge in waste treatment. The utility of polymer pendant ligands for the extraction of a variety of metal ions from aqueous media has been previously investigated.
Removal, immobilization, and characterization of mercury in waste streams, such as those generated at U.S. Department of Energy facilities, are a major remediation objective. These waste streams can contain high levels of mercury and include aqueous and non-aqueous sludges, absorbed liquids, and partially or fully stabilized sludges. These streams require the removal and immobilization of mercury to effectively meet environmental criteria for disposal. Removal of mercury ions, in particular, requires a robust and selective sequestering agent, since such waste streams typically are not only very acidic, but also contain a wide variety of other heavy metal ions, such as Al3+, Fe3+ and Pb2+. Thus, there is a need to develop polymeric extractants for heavy metal ion remediation in mixed waste streams. The present invention provides an effective method for mercury removal, particularly selective mercury removal, from acidic waste streams.
The present invention is a method to remove heavy metal cations, particularly mercury, directly from acidic aqueous waste streams using a novel composition having pendant crown thioethers (thiacrowns) attached to polymer supports. These thiacrown ligands or polymers are well-suited for extracting mercury due to the high affinity that sulfur crowns have for the Hg(II) ion, and they are robust enough to hold up to the acidic conditions of the mixed waste streams.
A novel series of thiacrowns can be synthesized by a method of the invention wherein hydroxyalkyl group (e.g., hydroxymethyl) pendant arms are incorporated into a thiacrown precursor, the precursor is cyclized to form the thiacrown, the hydroxyalkyl group of the pendant arms is converted to nitrogen-containing group (e.g., amino group), and the resulting macrocycles are allowed to be attached to polymer supports (via the nitrogen-containing pendant arms) to form xe2x80x9cthiacrown polymer compositionsxe2x80x9d that have exhibited highly effective mercury extraction from the aqueous waste streams. The thiacrown polymers are synthesized by making acyclic thiacrown precursors containing a side chain (e.g., pendant) moiety that is unreactive during the cyclization (i.e., ring closure) step, but can then be functionalized (for attachment to the polymer) following the ring closure. The crown precursors can contain both sulfur and oxygen heteroatoms and the resultant thiacrowns typically contain from 3 to 6 sulfur atoms.
Such a mercury removal method is effective in the treatment of industrial wastewater, especially in mixed waste streams containing additional metal ions such as iron, cadimium, lead and aluminium where a selective process for removing mercury is required. An example of the utility of the process of the invention is at the Idaho National Engineering Laboratory, which produces and disposes of submarine reactor fuel rods. In the disposal process of these fuel rods, mercury is added in catalytic amounts to an acidic aqueous waste stream, which results in levels of mercury of 100 ppm or more. To dispose of the waste properly, the mercury levels must be reduced. A large volume (millions of gallons), a corrosive nature of the waste stream, as well as a competitive binding of other metal ions in the waste, combine to pose a complex remediation problem. The thiacrown polymer composition of the present invention has the extraction capability to remove mercury from these acidic fuel reprocessing waste streams.
Other advantages of the present invention will become apparent from the following description and accompanying drawings.